1. Field of the Invention
This invention relates to a gradation record printer in which, as in a heat-sensitive printer using heat-sensitive sheets, or in a sublimation type printer using a sublimation type ink sheet and an ink receiving sheet, the amount of energy applied to a thermal head is controlled to print an image on a printing medium with gradations, and more particularly to an improvement of the printer that gradations can be reproduced satisfactorily with printing media such as heat-sensitive sheets and ink sheets different in thermal characteristic.
2. Description of the Related Art
FIG. 6 is a block diagram showing the arrangement of a conventional gradation record printer disclosed, for instance, by Unexamined Japanese Patent Publication No. Sho. 63-19273. FIG. 7 is a graphical representation indicating gradation numbers with amounts of energy applied to the thermal head (sixty-four (64) gradations). Here, the gradation number is defined as a number indicating a gradation of an image signal. In FIG. 7, the gradation numbers of zero through sixty-three are used to express the sixty-four gradations. In general, printing media are heat-sensitive sheets, or ink sheets and printing sheets. Those sheets have their own thermal characteristics, depending on their kinds and manufacturers.
In FIG. 6, reference numeral 1 designates a platen; 2, a thermal head; 3, a printing sheet; 4, a ROM (read-only memory) for storing feed (current application) time width data for setting the amounts of energy which are to be applied to the thermal head 2 with respect to gradation numbers; 5, a gradation counter for producing gradation numbers successively beginning with the gradation zero; 6, a feed pulse generating circuit for forming feed pulses in correspondence to feed time width data; 7, a printing control circuit comprising a shift register 7a, and a group of latch circuits 7b; and 8, a comparator which compares the gradation number of an input image signal with the gradation number provided by the gradation counter 5, and applies the feed time width data of the ROM 4 to the feed pulse generating circuit 6 with the coincident gradation number as the address number (read address) of the ROM 4. In the gradation record printer thus organized, the feed time widths corresponding to the gradation numbers are stored as data in the addresses in the ROM 4 whose address numbers correspond to the gradation numbers in the relation between the feed time width and the gradation number as indicated by the curve C in FIG. 7.
The operation of the gradation record printer thus constructed will be described. In the comparator 8, the gradation number of a picture element of an input image signal is compared with a gradation number provided by the gradation counter 5, so that, with the gradation number equal to the gradation number of the image signal as a read address, the feed time width data of the address is read out of the ROM 4 and is applied to the feed pulse generating circuit 6. The circuit 6 generates a feed pulse in correspondence to the feed time width data. The feed pulse is generated for every picture element and applied to the printing control circuit 7, where the feed pulses are combined with the synchronizing signal provided through the comparator 8 into a printing signal for one line. The printing signal thus formed is applied to the thermal head 2, to start a printing operation.
In the conventional gradation record printer thus organized, the relationship between the gradation number and the feed time width (or the amount of energy to be applied) is, in general, established in accordance with the thermal characteristics of a printing sheet which is most frequently used. In this case, the relationship between them established is as indicated by the curve C in FIG. 7.
In this connection, let us consider the case where a different printing sheet is used whose thermal characteristic is as indicated by the curve D in FIG. 7. If, in this case, the gradation number is n.sub.2, the feed time width is smaller by .DELTA.E.sub.2 in the case of using a printer set in accordance with the curve C. That is, the amount of energy is insufficient, and the resultant density is not acceptable. If the gradation number is n.sub.1, the feed time width is larger by .DELTA.E.sub.1. That is, the resultant density is larger than the predetermined value. Hence, in the printing operation, the gradations reproduced are unsatisfactory.